The production of yeasts assumes the supply of different nutritive elements capable of ensuring their growth. These different nutritive elements are in particular carbon, nitrogen, phosphorus, sulphur, minerals and vitamins. Among the natural media which can be used for the production of yeast, molasses occupy a special position. In fact, molasses constitute a relatively complete medium capable of ensuring the supply of these nutritive elements. The average composition of the molasses is as follows: 66% to 73% sugars, 15% to 23% organic compounds and 10% to 12% mineral compounds (in percentages of total dry matter). The carbonaceous source provided by the molasses is essentially constituted by saccharose or sugars resulting from its hydrolysis (glucose and fructose).
However, the increased scarcity of cane and beet molasses, in particular linked to their extensive use for the production of bioethanol, jeopardizes this type of production. It is in particular necessary to find novel carbonaceous substrates to replace the saccharose provided by the molasses.
The production of yeasts can also be carried out on glucose or fructose syrups, for example obtained from the hydrolysis of starch and originating in particular from cereals (maize, wheat, rice) or potato. However, these syrups are relatively expensive and do not represent an economically profitable solution for replacing the saccharose of molasses within the context of industrial yeast production.
Yeast of the genus Saccharomyces is capable of synthesizing glycerol under anaerobiotic conditions or in a situation of severe stress, for example in the case of osmotic or thermal stress. The yeast Saccharomyces is moreover capable of degrading glycerol, but in a purely oxidative metabolism and in the absence of catabolic repression by sugars. Thus, the yeast Saccharomyces, placed in the presence of glycerol and sugar in a batch culture, is not capable of consuming the glycerol, while sugar remains in the medium.
Moreover, in an oxidative metabolism, the yeast Saccharomyces cultured on a carbonaceous substrate constituted solely by glycerol consumes the glycerol extremely slowly, which is not compatible with yeast production on an industrial scale. (Lages and Lucas, 1997, Biochimica and Biophysica Acta).
There is therefore a real need for novel strains for the production of yeasts which make it possible to totally or partially replace sugar as a carbonaceous substrate, with yeast production yields compatible with economic and industrial exploitation, and/or without loss in quality of the yeasts produced.